In order to meet the Brillouin frequency shift requirement of dual channel laser source for Brillouin optical time domain analysis(BOTDA) system, an optical fiber Brillouin frequency shifter is designed and placed in the probe pulse channel to make Brillouin frequency shift difference(about 11.2 GHz) between the probe pulse light and the frequency scanning light in another channel. By analyzing the frequency shifter output spectral linewidth narrowing principle, the Brillouin Stokes spectral output with narrow linewidth is formed by combining optical fiber stimulated Brillouin gain spectrum and specific ring cavity structure. The two performance indexes, including threshold of stimulated Brillouin scattering of ring cavity frequency shifter and light-light conversion efficiency, and their influence factors are analyzed. An experimental device of Brillouin frequency shifter with a seed source of 1551 nm and 9 km single mode fiber is set up. The experimental results indicate that the threshold value of stimulated Brillouin scattering is 2.3 mW and the wavelength change corresponding to the Brillouin frequency shift is about 0.1 nm, when the coupling ratio of directional coupler is 0.4 and light-light conversion efficiency is 49%. This optical fiber Brillouin frequency shifter meets the basic light source technical indexes of BOTDA system, which reduces the complexity and the cost of the system.